The present invention relates generally to oxygen sensors. More particularly, the present invention relates to a method for applying and controlling current to a pumped air reference oxygen sensor.
Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, oxygen sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases. More specifically, oxygen sensors may be used to sense when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric oxygen sensor generally includes an ionically conductive solid electrolyte material, a porous electrode on the sensor""s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor""s interior surface exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria stabilized, zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:   E  =            (                        -          RT                          4          ⁢          F                    )        ⁢          xe2x80x83        ⁢    ln    ⁢          xe2x80x83        ⁢          (                        P                      O            2                    ref                          P                      O            2                              )      
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
pO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture. Increased demand for improved fuel economy and emissions control has necessitated the development of oxygen sensors capable of quantifying the exhaust oxygen partial pressure over a wide range of air fuel mixtures in both fuel-rich and fuel-lean conditions.
Conventional sensors use two types of air reference electrodes. The first type has a sizeable air chamber to provide oxygen from an ambient air supply to the reference electrode. However, to avoid contamination by the exhaust gas, the air chamber requires a hermetic seal sensor package, which is expensive and is problematic in field applications. The second type is a pumped-air reference electrode. It uses a pump circuit to pump oxygen from the exhaust gas to the reference electrode. As such, it does not require a sizeable air chamber connected to ambient air. Nor does it require a hermetic seal sensor package.
Pumped air reference oxygen sensors are advantageous over sealed oxygen sensors, as the latter are subject to air reference contamination. However, there are also some drawbacks associated with pumping current applied to pumped air reference oxygen sensors, such as the internal resistance of the sensor. During xe2x80x9ckey onxe2x80x9d of the system, a DC offset voltage is introduced on the sensor output signal that, in turn, increases the system light-off time. In addition, pumped air reference sensors may also be susceptible to air reference contamination following engine shut down and during a subsequent start up.
It is therefore desirable to provide a method of controlling the pumping current applied to a pumped air reference oxygen sensor that addresses the aforementioned concerns.
The problems and disadvantages of the prior art are overcome and alleviated by a method for applying and controlling current applied to an air reference oxygen sensor included in a vehicle exhaust system. In an exemplary embodiment of the invention, the method includes measuring an output voltage across the oxygen sensor when the exhaust system is initially activated and applying a current through the oxygen sensor when the output voltage reaches a value determinative of light off of a catalyst within the exhaust system. The magnitude of the applied current corresponds to a predefined purge value.
In a preferred embodiment, the method also includes monitoring an exhaust temperature in the system and decreasing the magnitude of the current applied through the oxygen sensor from the predefined purge value if the exhaust temperature indicates that the exhaust system is operating at a first condition. Conversely, the magnitude of the current applied through the oxygen sensor is increased from the predefined purge value if the exhaust temperature indicates that the exhaust system is operating at a second condition.